Rotary blowout preventers are commonly used in the petroleum industry to isolate wellbore fluids while drilling procedures are being conducted. Typically a casing spool having a discharge portal thereon is provided for the wellbore fluid to exit through. The rotary blowout preventer is connected to and supported on the casing spool and receives a drill string therethrough which is rotated to facilitate drilling of the wellbore.
One method for rotating the drill string is to extend an elongated, cross-sectionally polygonal kelly through an engine driven rotary table housed in the drill deck. The table has a polygonal bushing orifice therein through which the kelly is received. The kelly is connected to the uppermost joint of drill pipe forming the drill string to rotate the same under the rotating influence of the rotary table. Rotary blowout preventers are provided that can sealingly engage the kelly while it rotates. One such blowout preventer is disclosed in U.S. Pat. No. 3,492,007 issued to Jones on Jan. 27, 1970. Jones provides a hexagonal split kelly bushing for gripping a hexagonal kelly. The kelly bushing is connected to a rotary housing in the blowout preventer to secure the housing to the kelly for concomitant rotation therewith. The rotary housing carries an elastomeric packer assembly therein that sealingly engages the kelly. The packer assembly rotates with the rotary housing and is not subjected to rotary forces from the kelly because of the hexagonal kelly bushing's connection to the rotary housing. Such connection promotes concomitant rotation of both the rotary housing and the packer assembly with the kelly. Without connection to the kelly bushing, the kelly would rotate relative to the packer assembly and would wear or otherwise damage the elastomeric packer, requiring replacement thereof.
A second method for rotating the drill string is to use an overhead drive connected to an uppermost section of drill pipe for rotating the same. No kelly is used so one section of the cylindrical drill pipe is always positioned within the rotary blowout preventer during rotation of the drill string. The cylindrical nature of drill pipe presents a problem for conventional blowout preventers since a rigid bushing that will engage the pipe's cylindrical surface and still permit the longitudinal movement of the drill pipe through the bushing is unavailable in the industry. Furthermore, each drill pipe has an expanded diameter collar on one end to facilitate connection thereof with the next adjacent drill pipe. Longitudinal movement of the drill pipe through a rigid bushing would be prohibited by the expanded diameter collar.
As previously mentioned, direct contact of the elastomeric packer assembly with a rotating kelly or drill pipe will result in rapid wear or even spontaneous disintegration of the packer. As packers such as the inner packer shown in Jones are relatively expensive and time consuming to replace, direct contact thereof with the drill pipe is not advised.
Additionally, the seals and bearings commonly found in rotary blowout preventers are particularly susceptible to wear from heat generated by the temperature of wellbore fluids and the friction commonly occurring with such rotary bearings and seals. Foreign particulate matter suspended in the rotary blowout preventer is also a common element promoting the wear of such seals and bearings. Once the seals and/or bearings have been worn, they must be replaced. As shown in Jones, the seals and bearings are commonly seated deep within the outer casing of the blowout preventer and require substantial effort and time to replace.